oeqa/context: Only set buffer mode for non-concurrent tests

Periodically we'd see:

NOTE: core-image-sato-1.0-r0 do_testsdk: ======================================================================
NOTE: core-image-sato-1.0-r0 do_testsdk: ERROR: broken-runner
NOTE: core-image-sato-1.0-r0 do_testsdk: ----------------------------------------------------------------------
NOTE: core-image-sato-1.0-r0 do_testsdk: testtools.testresult.real._StringException: Traceback (most recent call last):
  File "/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-mips/build/meta/lib/oeqa/core/utils/concurrencytest.py", line 122, in _run_test
    test.run(process_result)
  File "/usr/lib/python3.6/site-packages/subunit/__init__.py", line 1194, in run
    protocol = TestProtocolServer(result, self._passthrough, self._forward)
  File "/usr/lib/python3.6/site-packages/subunit/__init__.py", line 514, in __init__
    stream = stream.buffer
AttributeError: '_io.StringIO' object has no attribute 'buffer'

which seems to occur if a result arrives before all the runner threads
have started. The runner's result handling changes sys.stdout to a buffer
temporarily which can be seen in other threads and it can sometimes fail.

Since the tests are running in a separate process we don't need this buffer
handling in the concurrent case so only set when not parallelising. The
concurrent class handle setting buffer mode internally.

(From OE-Core rev: e2c66aa0f74d19fbc7e56af1fb440ad6473d3a8e)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/core/context.py

@@ -58,7 +58,7 @@ class OETestContext(object):
         self.suites = self.loader.discover()
 
     def runTests(self, processes=None, skips=[]):
-        self.runner = self.runnerClass(self, descriptions=False, verbosity=2, buffer=True)
+        self.runner = self.runnerClass(self, descriptions=False, verbosity=2)
 
         # Dinamically skip those tests specified though arguments
         self.skipTests(skips)
@@ -70,6 +70,7 @@ class OETestContext(object):
             concurrent_suite = ConcurrentTestSuite(self.suites, processes)
             result = self.runner.run(concurrent_suite)
         else:
+            self.runner.buffer = True
             result = self.runner.run(self.suites)
         self._run_end_time = time.time()